Aromatic polyimides have heat resistance which is of the highest level among polymeric substances and also exhibit superior mechanical characteristics, electrical characteristics, and the like. For these reasons, aromatic polyimides are used as a raw material in a wide range of fields, including aerospace and electrics/electronics.
However, aromatic polyimides generally have poor processability, and thus are particularly unsuited for use in melt molding and for use as a matrix resin in a fiber-reinforced composite material. For this reason, in cases where an aromatic polyimide is to be used as a matrix resin for a fiber-reinforced composite material, a polyimide capable of a thermal addition reaction is typically used. Specifically, a low-molecular-weight imide oligomer which has been terminally modified by a thermally-crosslinkable group is impregnated into fibers and then the resin is crosslinked and cured in a final step. In particular, an imide oligomer using 4-(2-phenylethynyl)phthalic anhydride as a terminal capping agent is known to provide excellent balance in terms of the moldability, heat resistance, and mechanical characteristics of a composite material.
For example, Patent Literature 1 discloses a terminally modified imide oligomer which is (a) synthesized from raw material compounds including (i) one or more aromatic diamines including 2-phenyl-4,4′-diaminodiphenylether and (ii) a 1,2,4,5-benzenetetracarboxylic acid, and (b) terminally modified by 4-(2-phenylethynyl)phthalic anhydride.